Look in the bedding departments of places like WalMart, KMart, Target for foam mattress pads. You'll find 2" -3" eggcrate foam in your choice of sheets the size of twin, full, queen or king bed mattress. Fabric stores like JoAnn's Fabrics also carry foam in sheets to use for reupholstering chairs & couches.
If you're handy with scissors and a staple gun, you can slap together some 'gobos' -- a studio term for those baffles of various widths & heights you may have seen in photos or videos that get placed around drummers, guitar amps, etc. to isolate players' sound from one another in live recording sessions.
Get some 4'x8' sheets or pre-cut 2'x4', 4'x4' sheets of 1/2" 'pressboard', MFD, or whatever you call it that's used in place of expensive plywood at Home Depot, Lowe's, wherever. Pickup a few 2x4's. And a box of 3" #8 woodscrews. And a box of 1/4" staples for the staple gun. If you need a gun, Black & Decker or Arrow for T40 staples, at the Depot for around 20 bucks.
Cut the foam to size to cover one side of the board, staple it in place. Cut an old (or new) bedsheet or fabric of your choice big enough to cover the foamed side & overlap the edges, or go all the way around if you prefer. Wrap that over the foam and staple it in place.
Cut some 1' long lenghts for 'feet' from the 2x4. Screw these up through the 2x4 & into the 1/2" 'gobo' board, about 6" or so in from each end.
Voila.
For tall ones, like to make yourself a 'vocal' booth, get a couple of cheap 36" bi-fold doors instead of the MFD sheets. Use thinner (1") mattress pad sheets to cover these, so you can fold them to 90-degree angles and make a tall 'box'. Or just use one and face it into a corner of the room.
Here's a shot from my old house's home studio room 4 years ago. It was a big enough room to set up & record a 5 piece band, about 18x24, with 10' ceilings. (These days, I cram myself into half of a 10x12 den). To the right, behind the bongos, you can just about make out 2 of these tall bi-fold door 'gobo's isolating a corner of the room from the rest of the room, with a small condenser mic in the middle to record hand percussion, and a LD conenser vocal mic set up to the left to sing facing the corner, or kncok down to record acoustic guitar the same way...you want some reflection of hi-freq sounds or you end up with mud for sound:
You can make as many as you need, move 'em around, store 'em out in the garage, whereever when not in use.
You don't need to baffle the whole room; you really just have to baffle the sound source and cut it off from throwing sound off (or through) the walls of your room.
